With the wide use of radio telephone systems, the shortage of radio waves to be allocated has become a serious problem, therefore making it indispensable to develop techniques for advantageously using radio waves. Several techniques have been proposed, for example, digitizing the radio equipment, using higher frequencies, and using a narrower bandwidth as a radio channel step.
As one example of prior art, FIG. 1 shows a schematic block diagram of a conventional transmission frequency synthesizer in digital radio telephone equipment.
A reference signal source 1 generates a reference signal of a frequency R times the channel step frequency f.sub.CH, where R is an integer of one or more. A phase lock loop (PLL) synthesizer 2 generates a signal of a frequency f.sub.LO selected from channel frequencies in accordance with the channel designation data. This frequency is represented by f.sub.LO =f.sub.0 +M.multidot.f.sub.CH, where f.sub.0 is a base frequency corresponding to channel No. 0, and M is an integer within the range 0&lt;M&lt;(m-1) according to the channel designation data, where m is the total number of channels.
A modulated signal source 3 outputs a periodic signal of a fixed frequency f.sub.M. A modulator 4 modulates the periodic signal input from the modulated signal source 3 by using a modulating wave to be transmitted. A mixer 5 carries out frequency addition (or subtraction) of the signal of the frequency f.sub.LO input from the PLL synthesizer 2 and the signal of the fixed frequency f.sub.M input from the modulator 4 and outputs a signal of a frequency f.sub.TX as a transmission wave. For example, the frequency of the transmission wave of channel No. 0 is represented by the following expression: EQU f.sub.TX =f.sub.LO (=f.sub.0 +O.multidot.f.sub.CH)+f.sub.M =f.sub.0 +f.sub.M.
In the conventional frequency synthesizer shown in FIG. 1, since the output frequency f.sub.M of the modulated signal source 3 is fixed, selection of a transmission frequency from channel frequencies is made by changing the output frequency of the PLL synthesizer 2. Therefore, when the number of channels, m, is increased as much as possible in order to advantageously use the radio waves, it is necessary to use a higher frequency band as the transmission frequency f.sub.TX or to use a narrower bandwidth as a radio channel step. This makes the number of frequency divisions in the PLL synthesizer 2 undesirably large, resulting in much time to phase-lock a desired channel frequency.
Further, an apparatus in which a coarse resolution PLL loop is combined with a fine resolution PLL loop is disclosed in the U.S. Pat. No. 5,028,887 (lines 3 to 13 of the second column). However, this configuration is capable of providing a frequency step over only one broad frequency range, not two frequency ranges as necessary to a transmitter and receiver such as a radio telephone apparatus. If two frequency ranges are to be covered by this conventional apparatus, two PLL synthesizers must be provided for each of two frequency ranges, which undesirably contributes to increases in hardware and power consumption.
Furthermore, employing a digital direct synthesizer (DDS) in place of the PLL synthesizer 2, the circuit configuration is disadvantageously complicated resulting in increased power consumption.